the primary goals of this work are to define the mechanisms of recombination of the double-stranded (ds)RNA bacteriophage phi 8 and its relatives in the cystoviridae. Comparison of the properties of phi8 and its previously isolated relative phi6 indicate that the viruses differ markedly in abilities to acquire foreign RNA into their genomes both in terms of uptake and recombination. Whereas phi6 and its close relatives are capable of heterologous recombination, phi8 is capable of both heterologous and homologous recombination. The differences in promotion of heterologous and homologous recombination are not well understood in any RNA virus system; this is the first demonstration of homologous recombination in the dsRNA viruses. The greater versatility in recombination of phi8 will make it possible to elucidate mechanisms of recombination in the cystoviridae by genetic and biochemical approaches. The phi8 system is a powerful tool to investigate these mechanisms through the use of reverse genetics, in vitro replication and recombination experiments and manipulation of the RNA polymerases of phi8 and its relatives. Phi8 and its relatives are models for the replication and assembly of the eukaryotic dsRNA viruses. Several members of the reoviridae, e.g. rotavirus and blue tongue virus, are the etiologic agents of important human and animal diseases. Since reverse genetic techniques and in vitro genomic packaging are not available for these viruses, the cystoviridae (dsRNA bacteriophages) offer a valuable model for system for the study of the eukaryotic systems.